Design of Sliding Mode Controller for Field Controlled DC Motor Under Parameter Uncertainties

Author Name(s): K. Anusha, B. Amarendra Reddy, M.Ramesh
Author Email: anushakanu95@gmail.com

Abstract

Designing the speed controller for a field controlled DC motor with uncertain parameters is a challenging task. Sliding Mode Control technique is widely used robust controller design methodology that takes care of uncertain parameters. Here, in this paper, first order sliding Mode controller is designed to regulate the speed of a DC motor using unit vector approach. The effectiveness of the control strategy in regulating the dynamics of the field controlled DC motor is verified in simulation in the MATLAB environment. Each parameter of this field controlled DC motor is varied by 50% and performed simulations.

Introduction

DC motors are generally used in various industrial applications such as rolling mills, mining, traction, power plants, conveyor belts, paper industries etc… The usage of the DC motors spread from low horse power to multi-megawatt due to its high power, torque, speed ranges, high efficiency, and fast response [1]. In industrial process control sometimes it is necessary to be able to adjust motor’s speed over a wide range with good speed and robust control. Therefore, it is an important task to design a controller for regulating the speed of DC motor.

Sliding mode control technique is inherently a robust control technique and it is used to regulate the speed of the DC motor control. The features of sliding mode control are order reduction, disturbance rejection, strong robustness. This sliding mode control is also known as variable structure control system which has been utilized in the designing of robust regulators, model reference systems, and adaptive schemes, tracking systems, state observers and fault detection schemes.

The procedural steps in designing SMC strategy  are enumerated as follows.

1) Determining sliding surface that governs the system dynamics.

2) Defining the control law. The function of the control law is to attract the state trajectories towards the sliding surface [5] and also to maintain the trajectories on the sliding surface. This paper is organized as follows. (i) Mathematical modelling aspects of the field controlled DC motor is discussed in the section II. (ii) Sliding mode Control law aspects are discussed in the section III. (iii) Control strategy derivation for field controlled DC motor is dicussed in section IV. (iv)Simulation studies using MATLAB are discussed in section V.   (iv)Conclusions are presented in section VI.

Conclusion

The paper dealt with an approach of first order sliding mode control applied to field controlled DC motor. Detailed modeling aspects of the field controlled DC motor are discussed. Detailed SMC design aspects under parameter variations are discussed.  As each parameter of this field controlled system is increased by 50% and the same parameters are decreased by 50% then the variation of responses for the state variables angular position, angular velocity and field current  are obtained. The history of all  state  variables with time are plotted under variations of parameters. Control signal and switching surface with time are plotted under parameter variations.

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